1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An electrophotographic method-based image forming apparatus finds wide application in copying machines, printers, facsimile machines, or the like equipment. In general, image formation is accomplished in the following manner. Firstly, there is prepared a photoreceptor having a photosensitive layer containing a photoconductive substance formed on the surface thereof. After the surface of the photoreceptor is electrically charged uniformly, an electrostatic latent image corresponding to image data is formed thereon through a few different image-forming process steps. The electrostatic latent image is developed into a toner image with use of a developing agent containing toner supplied from development section. The toner image is directly transferred onto a recording medium such as a paper sheet, or transferred onto an intermediate transfer medium once, and is thereafter transferred onto a recording medium. On the recording medium, the toner image is heated and pressurized, and is eventually fixed thereon by a fixing section such as a development roller according to a heat fixing method.
An example of the image forming apparatuses of heat fixing has a configuration in which an intermediate transfer belt, eventually a toner image transferred onto the intermediate transfer belt is heated by extending the intermediate belt, which is an intermediate transfer medium, by heating rollers, and the toner image is transferred and fixed onto a recording medium while being heated (refer to Japanese Unexamined Patent Publication JP-A 10-63121 (1998), for example). The image forming apparatus of JP-A 10-63121 is characterized by consuming a relatively small electric power. However, in this image forming apparatus, the recording medium is not heated, and therefore when a toner image is brought in contact with the recording medium for transfer and fixing, the temperature of the toner image is reduced, so that the toner image may not be fixed or image deterioration due to offset or the like may occur. Moreover, an image forming apparatus is proposed in which the toner image on the intermediate transfer belt is heated and the recording medium on which the toner image has not been transferred or fixed yet is also heated, and the toner image is transferred and fixed on the recording medium while being heated (refer to Japanese Unexamined Patent Publication JP-A 2004-151626, for example). With the image forming apparatus of JP-A 2004-151626, the adhesion of the toner image to the recording medium is improved, but since not only the toner image, but also the recording medium is heated, a heating section with a large heat capacity is required, thereby increasing power consumption and resulting in a current situation in which the heating section consumes at least a half of the total power consumed in the image forming apparatus.
On the other hand, saving energy is a current trend to counter global warming, and electrophotographic image forming apparatuses are widely used. For these reasons, also in electrophotographic image forming apparatuses, there is a demand for reducing power consumption when the toner image is fixed onto the recording medium. Furthermore, in the heat fixing, as described above, the heating section is used inside the apparatus, and the inside of the apparatus reaches a high temperature, so that it is necessary to increase the heat resistance of the components thereof, thereby increasing the material cost. In addition, in the heat fixing, since fixing is not performed until the fixing portion reaches a predetermined temperature, and therefore a time until the predetermined temperature is reached, that is, a warm-up time is often necessary. Furthermore, the heat fixing has a problem in that fixing of a multicolored toner image onto a recording medium takes more time than fixing a monochromatic toner image. Therefore, there is a demand for shortening the time for fixing a multicolored toner image. In view of these demands, wet fixing employing a fixing solution containing water and a liquid that can be dissolved or dispersed in water and has an action of softening or swelling toner is proposed. In the wet fixing, a toner image that is softened or swollen by application of the fixing solution is attached to a recording medium, and pressed so that the toner image is fixed onto the recording medium. The wet fixing consumes the power much less than the heat fixing, and therefore the wet fixing is useful in view of saving energy. Furthermore, the time for fixing a multicolored toner image can be shortened, compared with the heat fixing, because a large amount of heat capacity is not necessary. Therefore, various further improvements of the wet fixing are proposed.
For example, there has been proposed a fixing apparatus in which a jet of fixer fluid ejected from a fixer fluid ejecting member having a plurality of pores is applied relatively to a toner image carried on an intermediate transfer medium or a recording medium, the fixer fluid being applied only to a part to which toner is attached and heated (refer to Japanese Unexamined Patent Publication JP-A 2004-109747, for example). That is to say, in this fixing device, after a fixing solution is applied to a toner image on an intermediate transfer medium or a recording medium, heating is performed. However, an unfixed toner image is merely an aggregate in which toner particles gather without being physically or chemically bonded at room temperature. Therefore, when a liquid such as the fixing solution is applied directly to an unfixed toner image, a flow or an aggregation of toner particles tends to occur before the toner particles are softened and/or swollen, and bonded firmly to each other. As a result, a blur is generated in the image edge after fixing, and non-uniformity occurs in a half tone portion that should be uniform, so that high definition images cannot be achieved. It is natural that even when heating is performed after flow or aggregation of toner particles occurs, the original state cannot be recovered. Furthermore, in the fixing device of JP-A 2004-109747, in the case of letting a toner image carried on the intermediate transfer medium, an intermediate transfer belt, which is the intermediate transfer medium, is subjected to water-repellent treatment such as a treatment with fluorine. Therefore, even when the fixing solution is applied to an area where a toner image is formed of the intermediate transfer belt, the fixing solution is present only in the portion where toner is attached (image portion) of the area where a toner image is formed without staying in the portion where toner is not attached (non-image portion) between the portions where toner is attached. However, when the fixing solution is applied only to the image portion in the recording medium, expansion and contraction occurs in the image portion, and does not occur in the non-image portion, so that wrinkles originating from the image portion are inevitably generated. In particular, when recording paper made from paper fiber with water is used as the recording medium, this tendency is significant. In the case where the minimum amount of the fixing solution necessary to swell toner is applied, this problem can be avoided. However, the minimum amount is very small, so that it is difficult to weigh the minimum amount accurately. Moreover, when the fixing solution is applied only to the image portion, toner attached to the non-image portion surrounding the image portion, for example, due to fogging remains on the recording medium without being fixed, so that the unfixed toner may soil hands or clothing.
In the image forming apparatus of JP-A 10-63121, it is possible to apply the fixing solution to a toner image when transferring and fixing the toner image on the heated transfer belt onto a recording medium that is not heated. However, a large amount of fixing solution is necessary in order to enhance the adhesion between the toner image and the recording medium and between the toner particles, in the case where heat is not supplied for transfer and fixing and there is no particular measure for that, as in JP-A 10-63121. When a large amount of fixing solution is used, wrinkles or curling is inevitably generated in the recording medium. Moreover, it is necessary to often supply the fixing solution or necessary to provide a large capacity tank for storing the fixing solution, resulting in poor maintainability or increase in the size of the apparatus. Furthermore, when toner images on resin recording media such as sheets for over heat projectors (hereinafter, referred to as “OHP sheets”), recording media having a resin layer on its surface or other media through which the fixing solution hardly permeates are fixed with the conventional wet fixing type image forming apparatus, the adhesion of toner becomes insufficient, and toner images may partially peel off.